I. Field of the Invention
This invention relates to the preparation of blowing agents and method of use thereof for foaming polyurethane. More particularly, the present invention relates to blowing agents having no known ozone depletion potential and having high dimensional stability. In addition, the present invention relates to a blowing agent that is usable in the production of a product having excellent fire retardancy.
II. Background of the Invention
There are several known blowing agents for the foaming of polyurethanes. One such agent is water. When water is used, a polyether such as polypropylene glycol is treated with a diisocyanate in the presence of some water and a catalyst (amines, tin soaps, organic tin compounds). As the polymer forms, the water reacts with the isocyanate groups to cause crosslinking, and also produces carbon dioxide which causes foaming.
The resulting foamed polyurethane was then covered with a polyvinylchloride synthetic material to create, for example, automobile components such as dash boards, handles and bumpers. This is a two-step process--the creation of the foam and the addition of the cover.
While this two-step process has been effective, it is time-consuming. As an alternative, a one-step process was developed in which an integral skin was formed on the polyurethane foam. However, in the one-step process, conventional blowing agents such as water did not work effectively because the skin did not form properly, and alternative agents were necessary.
The blowing (and bonding) agents of choice, until 1979, have been chlorofluorocarbons. This group comprises any of several compounds comprised of carbon, fluorine, chlorine and hydrogen, were commonly used as blowing agents. While safer and more efficient than other propellant gases, their use was prohibited in that year because of their depleting effect on stratospheric ozone. The most popular of these gases were dichlorodifluoro-methane (CFC-11) and dichlorodifluoromethane (CFC-12).
As alternates to the chlorofluorocarbons, other blowing agents such as the two-carbon hydrogen-containing halocarbons, or HCFC's. Such a composition and method are disclosed in U.S. Pat. No. 5,114,980 issued in 1992 to Lii et al. These alternatives have been found useful because they allegedly have lower ozone depleting characteristics. While an improvement over chlorofluoro-carbons, the HCFC's are still troublesome because they nevertheless deplete the ozone, even though to a lesser degree. The phaseout of such chemicals is inevitable because of their environmentally destructive character.
Other blowing agents that do not have ozone-destroying effect are known. One such common alternative is methylene chloride, and another known blowing agent is methyl formate, derived by heating methyl alcohol with sodium formate and hydrochloric acid. Also, pentane is known for use in expanded polystyrenes.
However, while offering advantages with respect to the minimal harm on the environment, these alternatives suffer from product shrinkage. Specifically, after the article is created and the foam is allowed to set, shrinkage begins almost immediately after setting. This is especially true in the case of products manufactured from enriched foams.